Just for information BAXI 105E

Yes Zoom, but the pump circulates the heated water through the heat exchanger. 

I read this thread with interest.  My Vaillant boiler (coming up for 10 years old) has had a couple of intermittent lockouts with an F62 fault.  The book says this is a gas valve fault, but further research shows it's also caused by a PCB fault, and if the temperature display jumps about (which mine does) then it's a PCB fault.

I suspect it's likely to be a dry joint or a power supply component that's failing.  Not sure I can be bothered to trace it so I have bought a replacement online (brand new, Vaillant original, around £100) and will fit it this weekend.

I was a bit nervous about the Gas Safe ‘legalities’ but I won't be going near the gas systems.  I may get the current settings off the existing board so if anything is not factory default then I can replicate it and all should hopefully be good.

Some of the online ‘advice' for this fault is terrible! E.g. ‘A new PCB will be more than £500 + fitting +VAT, so you may as well replace the boiler'.  All sorts of ‘call a registered engineer and be prepared for a bill’ stuff as well.

I consider myself competent to do a PCB replacement, provided it doesn't cause all the settings to go out, but if it does then I'll just need to get the ‘service’ done a bit early - it's due next month anyway.

Jason.

I think many Gas Safe ‘Engineers’ are of the simple photocopier service engineer mentality - monkey engineering - if that pcb doesn't work, swap it, then if that doesn't solve the problem, then swap the next and the next and the next until it does work. There are some good plumbers out there and there are some good design engineers out there but it is rare for a plumber to be both.

Being a nosey git, I would love to learn how the gas valve is modulated. I assume there is a sensor and a chip involved. It isn't like our old industrial gas furnace control - a few thermocouples and a full-on or full-off gas solenoid on each of the12 burners.

The older your boiler gets, the cheaper the spares become - I wouldn't have paid £100 for a pcb for an old boiler though.

Hope it works out for you.

being a nosey git, I would love to learn how the gas valve is modulated. I assume there is a sensor and a chip involved.

well, there always is a chip or 2 these days. Actually a lot of the modulation in modern boiler is really done by a changing the fan speed of the air, and one or 2 gas jets are then solenoid bang bang control but the varying airflow means the gas jet or jets  is/are poking into a variable degree of vacuum so more or less gas is sucked out depending on the airspeed. This is not unlike the petrol uptake from the jets in a carburettor varying as the throttle opens, and with care mixture can then be optimised over quite a wide range. - I assume you remember adjusting cars with those.

Truly modulating gas valves are rather rarer in a domestic boiler,  but behave like little more than a motorised gas tap Internally usually either a needle or butterfly valve with a closed loop feed back from a position sensor and a servo loop to compare the degree of valve opening with a control voltage or pulse-width modulated stream. Unless it has changed recently, remote control plane actuator servos work on the same lines.

Mike.

Grumpy: 
 

Yes Zoom, but the pump circulates the heated water through the heat exchanger. 

With the Alpha model that I referred to, the hot water diverter valve is purely mechanical with a spring loaded diaphragm operated valve and a micro switch. I presume that the valve is biased to heating and changes over when a mains water pressure is detected inside it. So the pump runs constantly when the boiler fires up and there is demand. What alters in the boiler when the room 'stat is satisfied to stop heating the rads?

Z.

 

The older your boiler gets, the cheaper the spares become - I wouldn't have paid £100 for a pcb for an old boiler though.

 

But would you pay £50.00 W.H.J?

What alters in the boiler when the room 'stat is satisfied to stop heating the rads?

In principle that microswitch on the diaphragm valve switches the gas & pump between the room stat/clock control and definitely on (when there's DHW demand).

(In practice it's a bit more complicated as the whole sequence will be overseen by the PCB - first starting the flue fan, then checking by an air pressure switch that it's working, then starting the pump, then opening the gas valve a little, firing the ignition, checking that a flame has been created, then fully opening the gas valve and so on, at each stage possibly aborting if things don't go smoothly. Likewise on shutdown it'll probably keep the fan flue running for a short while to clear the combustion gasses and then the pump for a bit longer to stop the primary heat exchanger from kettling.)

   - Andy.